In oceanographic work, submarine detection, and other applications, it is desirable to be able to lower instruments from a ship, oil rig, helicopter, or other station positioned at or above the water surface, preferably at a controlled rate and to a selected depth. Such instruments might, for example, make measurement profiles of physical properties of the ocean such as temperature, current velocities or salinity of the water, may collect water samples or samples from the ocean floor, may be used as a transport platform for equipment to detect submarines, fish, underseaflora, fauna, or rock formations, etc. While, particularly with the advent of microprocessors and memory chips having large capacities, the quality and sophistication of such equipment has been advancing rapidly, there have been few advances in the platforms for deploying such instruments and equipment.
Heretofore, such platforms have generally been of two types. One type of platform, which are referred to as disposables, are thrown from a ship, airplane or helicopter and return data to shipboard instruments or to airborne instruments via radio transmitters.
These devices have a number of disadvantages. First, they require a conductive cable which is fed out at the surface and/or from the platform as the platform descends. In some cases, such platforms permit readings to be taken only as the platform descends. For example, assuming that the depth of the water is greater than the length of the wire, the wire breaks when the platform reaches the end of the wire aborting any further transmission of data. In other cases, the platforms reach the end of their cable and continue to transmit data until the device either self destructs, is discarded, or wears out.
Since these platforms permit moving readings to be taken only while the platform is descending, there is no easy way to check the readings other than to deploy a second platform. The data which can be collected is also limited to data which can be quickly and easily transmitted and such platforms cannot, of course, be utilized for collecting water or soil samples or for other experiments which require that the instrument be recovered.
Finally, there is reluctance to use high cost instruments in a disposable platform, limiting the experiments which can be conducted using such platforms or probes, and/or making their use relatively expensive.
A second type of platform is a recoverable tethered platform wherein the platform with the instruments is lowered from for example a ship, the platform being attached to the end of a cable which cable is played out from a suitable level wind winch mandrel on the ship. The platform is retrieved by reversing the direction of rotation of the mandrel to rewind the cable on the mandrel.
These systems also have a number of disadvantages. One of the problems with existing systems where the cable is fed from the ship is superposition of the dynamics of the rolling surface vessel on the dynamics of the descending instrument. This causes non linearities in the readings obtained which, because of their somewhat random nature, are difficult to compensate for. In high seas, the motion of the system becomes a complex combination of different component motions, particularly as the platform approaches its terminal descent velocity. The cyclic motion at the surface is superimposed on the descent rate of the cable and platform. Typically the platform has a non uniform shape which due to fluid drag results in a lesser terminal velocity than that of the cable. When the downward cyclic component of motion adds to the descent rate it may temporarily result in slack cable between the platform and cable or between the cable and ship. This may result in catastrophic cable failure.
Further, such systems generally employ metal cable and are thus relatively heavy in the water requiring relatively large and relatively high powered equipment to both deploy and recover the platform. The size and weight of such equipment may require the use of a larger, and thus normally more expensive, ship than if smaller and lighter equipment could be utilized.
Since ship time is very expensive, it is often desirable that the time required to take a single set of readings be substantially reduced by increasing the fall rate of the platform from 1 to 2 meters per second to something in the 6 meter to 10 meter per second range. In the case of 6000 meter deep ocean experiments this could reduce the time for an experiment from 3 hours to 1/2 hour, with significant resulting cost savings.
It is therefore a primary object of this invention to provide a recoverable oceanographic platform which will substantially reduce the on station time required for deployment and recovery.
A more specific object of this invention is to increase the rate at which the platform may descend by removing limitations due to cable drag and to system dynamics such as the problem of cyclic motion of a very large mass-momentum (ship-cable platform) system.
Another object of this invention is to provide a recoverable tethered platform for oceanographic use which substantially isolates the platform from the dynamics of the vessel, thus eliminating the somewhat random errors in readings which may occur in some prior art systems as a result of such dynamics.
Still another object of the invention is to reduce the size, weight and cost of the on vessel equipment utilized for a recoverable tethered platform thereby permitting a smaller vessel to be utilized for such deployment.
While in the discussion above, it has been assumed that the ship or other station from which the instrument platform is being deployed is stationary, similar problems arise where the ship or other station is moving at a speed of up to for example 10 knots during deployment and recovery.